The present invention relates generally to process sensor systems, and more particularly to thermowell sensor housings for fluid sensors in industrial process monitoring systems.
Industrial process transmitters and sensor assemblies are used to sense various characteristics of process fluids flowing through a conduit or contained within a vessel, and to transmit information about those process characteristics to a control, monitoring and/or safety system remotely located from the process measurement location. Each process transmitter may be connected to one or more sensor and/or actuator assembly. Sensor assemblies may sense a variety of process parameters, including pressure, temperature, pH or flow rate. Process transmitters are typically electrically connected sensor assemblies via sensor wires used to transmit current- or voltage-based analog sensor output signals reflecting at least one such process parameter. Each transmitter reads these sensor output signals, and converts them into a measurement of the process parameter. Finally, the transmitter sends the information to the control system.
Sensor assemblies for sensing process fluid temperatures and changes in temperature commonly include at least one temperature sensor housed in a thermowell extending into the fluid flow. Thermowells are designed to be in physical contact with process fluids and to shield the temperature sensor from physical damage caused by direct contact with the fluid, e.g., impacts, corrosion, etc., while efficiently conducting heat between the fluid and the temperature sensor. Thermowell reliability is of critical importance in process monitoring, as broken or damaged thermowells can allow leakage of hazardous process fluids, and expose delicate and/or expensive sensors to process fluids. Severe thermowell damage can cause thermowells to detach, potentially causing further damage to downstream equipment.
Vibration is a principal cause of damage to thermowells and enclosed temperature sensors, making vibration damping and prevention critical to sustained operation of sensor assemblies in process fluids. Impingement of process flow on a thermowell creates turbulence in the process fluid via vortex shedding. This turbulence has a characteristic wake frequency fw determined by a plurality of factors, including the geometry of the thermowell and conditions and flow rate of the process fluid. Vortex shedding can cause harmful thermowell vibrations when wake frequency fw is close to fr, a natural resonance frequency of the thermowell. Accordingly, thermowells are often designed using predicted wake frequencies fw to avoid fw=fr resonance conditions. During the lifetime of a thermowell, however, changes in process conditions can cause shifts in fw and/or fr that increase the likelihood of resonance conditions.